This invention relates to a device which is adapted to be positioned in the path of a beam of electromagnetic radiation propagating in free space which changes characteristics of the beam. The invention is particularly, but not exclusively, concerned with microwave devices.
The term microwave refers to the part of the electromagnetic spectrum substantially in the frequency range 0.2 to 300 GHz. It includes that part of the spectrum referred to as millimetre wave (having a frequency in the range 30 to 300 GHz).
It is known from EP 0505040 to steer the direction of a microwave beam using a scanning device that comprises a body of ferrite material having a first magnetic coil along a first side of the body and a second magnetic coil along a second, opposite, side of the body. Each coil generates a magnetic field which passes through faces of the body. The device is configured such that magnetic field from each coil passes through the body in opposite directions. This causes a gradient in magnetisation across the body. The direction of the beam leaving the device is perpendicular to the gradient in the magnetic field across the body. Therefore the degree of deflection of the beam is controlled by the gradient in the magnetisation.
One application for such a device is in a control system for automatic or semi-automatic control of a vehicle. Such a control system comprises a radar system to determine location and speed of a vehicle relative to other vehicles and other objects or features, for example roadside furniture. This enables the control system to operate the vehicle automatically in cruise control and collision avoidance modes.
The radar system uses a microwave beam of narrow beam width having a half-power (3 dB) beam width of 2xc2x0 to 4xc2x0. A narrow beam width is used in order that, when reflected from other vehicles, a relatively strong usable signal is available for processing. Furthermore, a narrow beam width is necessary for angular location of obstacles. Whilst a narrow beam width is useful for vehicle speeds which are normal for road travel, for example 15 to 100 km/hour, it is less useful for manoeuvring a vehicle at lower speeds, for example when parking, reversing, negotiating obstacles and the like. Because the beam width is narrow it provides wide coverage at some distance from the vehicle but at distances close to the vehicle, there are blind spots on either side of the beam. Obstacles in the blind spots will not be detected which may be hazardous if the vehicle is under automatic control. Furthermore, at low speeds, or when the vehicle is stationary, it is relatively easy for pedestrians or other vehicles to move into the blind spots and become hazards to an automatically controlled vehicle. Although it is possible to use a beam steering device such as the one described above to scan the beam horizontally from side to side to search for hazards, beam steering cannot scan beams through large angles and blind spots will still be present. For these reasons it has been proposed that certain control systems disable their radar detection means when the vehicle is stationary or moving at slow speed. The control systems activate again when the vehicle moves off from stationary and reaches a pre-determined speed or when the vehicle accelerates to the pre-determined speed.
According to a first aspect the invention provides a device for changing the divergence of a beam of electromagnetic radiation comprising a body having an electromagnetic aperture for the beam and magnetic means arranged to generate a magnetic gradient across the body which acts on the beam as it passes through the body, characterised in that the magnetic means acts on the electromagnetic aperture such that there is a differential phase delay over the electromagnetic aperture from a centre region to a periphery region of the electromagnetic aperture.
Preferably the magnitude of the phase delay in the central region may be greater than the magnitude of the phase delay at the periphery region. Alternatively, the magnitude of the phase delay in the central region may be less than the magnitude of the phase delay at the periphery region.
The aperture may comprise at least one sub-aperture induced by activation of the magnetic means. Each sub-aperture may have a boundary defined by the magnetic means.
According to a second aspect the invention provides a device for changing the divergence of a beam of electromagnetic radiation comprising a body having an electromagnetic aperture for the beam and magnetic means arranged to generate a magnetic gradient across the body arranged to act on the beam as it passes through the body characterised in that the magnetic means comprises at least one element, a sub-aperture associated with each element and each element defines a boundary of its associated of sub-aperture, and the magnetic means acts on the electromagnetic aperture such that there is a differential phase delay over the electromagnetic aperture from a centre region to a periphery region of the electromagnetic aperture 28.
Preferably the magnetic means may be arranged to induce variations in the magnitude of magnetisation present across the aperture. The magnetic means may be arranged to induce variations in the magnitude of magnetisation present across each sub-aperture. For example, looking at a graph of magnetisation across the aperture or each sub-aperture there may be xe2x80x9ckinksxe2x80x9d, whether peaks or troughs, associated with each element of the magnetic means which is generating magnetic field. This applies whether an average value of gradient in magnetisation across the aperture has a positive or negative non-zero value or is zero. It may be these kinks which define the boundaries of the sub-apertures.
Preferably the magnetic means comprises one or more elongate sources of magnetic field. If there are a plurality of elongate sources, these may be disposed parallel to one another. Most preferably the magnetic means is one or more paths for carrying electric current. Conveniently the or each path is a metal wire. If there are a plurality of paths, current may travel in each path in a direction substantially parallel to the other paths. Alternatively current in some paths may travel in one direction and current in other paths may travel in an opposite direction. Current may travel in opposite directions in adjacent paths. Different amounts of current may be carried by adjacent paths.
Preferably the current carried by the or each path may be altered so as to change the magnitude of the magnetic field generated by the magnetic means and thus the magnetisation induced in the body. The current carried by the or each path may be switched on and off which may switch the beam between a wider beam width and a narrower beam width. Alternatively, the current may be varied in value between on and off states. As a result the degree of kinking in magnetisation across the aperture may be altered and the degree to which the device diverges or converges the beam (that is focusses or defocusses the beam) may also be altered. Individual paths (or groups of individual paths) may be controlled separately. For example, they may be switched on and off and varied independently of one another.
If the device is to diverge the beam in both azimuth and elevation directions, the magnetic means may be in the form of a grid comprising a first set of one or more elongate sources of magnetic field and a second set of one or more elongate sources of magnetic field in which the first set is orientated at an angle of greater than 0xc2x0 relative to the second set. Preferably the first and second sets are orientated at 90xc2x0 to each other. The magnitude of the magnetic field of the first and second sets of sources of magnetic field may be independently controllable in order to broaden the beam independently in azimuth and elevation directions.
The beam of radiation may be microwave radiation or may be millimeter wavelength radiation. Most preferably the beam of radiation is generated by a radar system.
The device may effectively be serving as a zoom lens for the beam of radiation. By zoom lens is meant a device which can diverge or converge the beam.
A magnetic material is one in which its internal magnetisation is effected by magnetic field. Preferably the magnetic material is an electrical insulator. It may be a soft ferrite. Ferrite materials may be particularly suitable since they combine high permeability with low conductivity and low losses. Due to the low conductivity, ferrite materials are easily penetrated by microwaves.
The magnetic means may be located adjacent one or more sides of the aperture. Preferably there are two magnetic means. The or each magnetic means may be a single wire or may be one or more coils. Preferably the magnetic means may be provided in one or more pairs on opposite sides of the aperture. If the device is configured to have one magnetic means presenting a North pole on one side of the aperture and the other magnetic means presenting a South pole on another side of the aperture this induces a positive or negative non-zero gradient in magnetisation across the aperture which can be used to steer the beam by an angle xcex8. Differential operation of the or each pair of magnetic means may change the value of angle xcex8. If the or each pair of magnetic means both present the same pole (whether North or South) on the sides of the aperture this induces a differential phase delay between a central and a periphery region of the beam as the beam passes through the device. In this way the device may change divergence of a beam without the need to provide separate magnetic means which, for example, divide the aperture into a plurality of sub-apertures. Alternatively, such aperture dividing magnetic means may be provided together with magnetic means.
Preferably there are two gradients in magnetisation which are in directions perpendicular to one another. This enables the direction of the beam to be controlled in azimuth as well as in elevation to achieve conical beam steering. In an embodiment in which the device is used in a surface vehicle (for land or water) elevation control means may still be required. For example, in a land vehicle elevation control may be required to compensate for braking which would cause the front of the vehicle to dip. It may also compensate for the effects of vibration. Elevation scanning may allow information to be gathered that can be used to identify roadside furniture and other objects such as bridges and the like.
The body may comprise a first material which contains at least one region of a second material having a magnetic permeability which is lower than the magnetic permeability of the first material. The or each region may extend from sides of the aperture or sub-apertures. The or each region may extend towards the centre of the aperture or sub-apertures. Preferably the or each region extends about two thirds of the way to the midpoint between the sides of apertures or sub-apertures. Preferably the presence of the or each region having relatively lower permeability causes more of the magnetic flux to be deviated away from the sides of apertures or sub-apertures and towards the centre of the aperture or sub-apertures than would be the case in the absence of the or each region.
Preferably the or each region comprises a slot in the first material containing the second material as an insert or as a filler. The or each slot may taper being thinner at an end nearest to the centre of the aperture or sub-apertures. Preferably the or each slot has a linear taper. Alternatively the taper may be curved.
According to a third aspect the invention provides a control system comprising a radar system which incorporates a device in accordance with the first or second or both aspects of the invention.
According to a fourth aspect the invention provides a vehicle incorporating a control system in accordance with the third aspect of the invention.
Preferably the vehicle is a land vehicle. Alternatively, the vehicle may be waterborne or airborne.